This invention relates to an imaging lens system, and in particular, to an imaging lens suitable for the condensing and collimation of a semiconductor laser or a light-emitting diode, and utilizing a distributed index lens.
A lens having an index distribution in a direction perpendicular to the optic axis is well known as a Celfoc lens (trade name) and is used as the reading system of a copying apparatus or as a connector for optical communications.
In recent years, attempts have been made to use such a distibuted index lens as the pick-up objective of a compact disk or the like. The forms of its use are disclosed, for example, in Japanese Laid-Open Patent Application Nos. 205122/1983, 62816/1984 and 62817/1984.
What is disclosed as a specific construction in Japanese Laid-Open Patent Application No. 205122/1983 is a lens system comprising a distributed index lens in which the refractive index becomes progressively lower substantially in a square distribution from the optic axis to the marginal portion and which has planar end surfaces, and a homogeneous convex lens separated from the distributed index lens by a finite distance. Also, the lens disclosed in Japanese Laid-Open Patent Application No. 62816/1984 is a lens system comprising a distributed index lens in which the refractive index becomes progressively higher from the optic axis to the marginal portion and which has planar end surfaces, and two to three lenses in which the refractive index becomes progressively lower from the optic axis to the marginal portion and which are cemented to or separated from the distributed index lens. The lens disclosed in Japanese Laid-Open Patent Application No. 62817/1984 is comprised of two distributed index lenses in which the refractive index becomes progressively lower from the center to the marginal portion and which are cemented together.
However, the ion exchange method is generally used to manufacture the above-described distributed index lenses according to the prior art, and this method basically utilizes heat diffusion and therefore, requires a relatively long time for the manufacture and has a severe problem in the manufacturing process, particularly for lenses of great aperture. Further, even if the lenses have an excellent index distribution shape from the viewpoint of aberration correction, a difference in manufacture occurs relative to the condition of actual natural diffusion and therefore, in many instances, the index distribution has been difficult to control by the ion exchange method. Also, some of the lenses disclosed in the aforementioned laid-open patent applications are designed with the application thereof to the objective of an optical pick-up system taken into account. Therefore, spherical aberration and coma are eliminated therefrom, but astigmatism remains in many instances. These lens systems could only be used up to the angle of view of the order of 3.degree.-4.degree.. That is, they could not be said to have a performance sufficient to be used in a collimating system or the like which required a greater allowance for the setting with a light source.
In view of the above-noted disadvantages peculiar to the prior art the present invention provides for an imaging lens which is easy to manufacture and efficiently achieves aberration correction and has such an imaging performance that permits the lens to be used even for a wide angle of view of 10.degree. or greater.
The imaging lens according to the present invention comprises a distributed index lens which has planar opposite end surfaces in which the index gradient near the optic axis is substantially zero and the refractive index becomes progressively higher from the optic axis toward the marginal portion. The plano-convex lens has a planar surface on the side thereof which is adjacent to the distributed index lens and is formed of a homogeneous medium, thereby achieving the above objectives.